VCSELs with integrated Photodiodes (VIPs) will be key components in automotive sensors for e.g. speed over ground measurements. Automotive applications require a wide temperature range for device operation (e.g. −40° C. . . . 120° C.). However, VIPs change their properties significantly over this wide range, if they operate at all.
The dependency of characteristic VCSEL performance parameters on temperature is well known in the literature (e.g. H. Li and K. Iga, “Vertical-Cavity Surface-Emitting Laser Devices”, S69ff, Springer 2003). Three major effects can be identified:
The cavity resonance shifts with the temperature in accordance with the relation:∂λres/∂T≈0.07 nm/K
Furthermore, the gain peak shifts in accordance with the relation:∂λgain/∂T≈0.32 nm/K
Additional effects are a change of the laser threshold and a decrease of the gain/current with increasing temperature.
A proper design allows operation of the VCSEL through a wide range of temperatures (e.g. −80° C. . . . 180° C. in B. Weigl et al., “High-performance oxide-confined GaAs VCSELs”, IEEE J. Sel. Top. Quantum Electron. 3, 409-415 (1997)).
However, the output power of such a device varies by more than a factor of 10 within the temperature range of operation, and the lasing wavelength is shifted by about 10 nm. In principle, the performance of the integrated photodiode (sensitivity and noise) also varies strongly with temperature. All of these effects are in conflict with the aim of stable VIP performance such as constant signal to noise, constant output power, which should preferably remain below the eye-safe limit and a well-defined wavelength. A 10 nm shift already causes a 1% systematic error in distance and velocity measurements if the laser is used as a self-mixing sensor.
The temperature-dependent wavelength shift of VCSELS has been employed to adjust the wavelengths of laser arrays for DWDMs. US 2008/0031294 A1 discloses a VCSEL array with individual wavelength adjustment of the VCSELs. The wavelength adjustment is achieved by bias heating. The array is mounted on a cooled surface. The wavelength is then individually set by heating elements in proximity with the VCSELs. However, the arrangement requires additional components near the VCSEL to achieve wavelength adjustment. Furthermore, the bias heating according to US 2008/0031294 A1 requires a cooled surface. Although the VCSELs are not cooled individually, the cooling requires additional components for heat dissipation.